Which of the following is directly related to deep-ocean currents?
a. density
b. wind
c. temperature
d. salinity

The answer is:  [A]:  "density" .

 Put simply,  deep-ocean currents are, in fact, "deep",  and dense water sinks to the bottom (think "depth") and the denser water (compared to the "less dense" water) continues to sink as the (comparatively) less dense water rises.

This is the best answer choice.
 
(Note:  Indirectly, "salinity" is related, since salinity and density are proportional;  since the "saltier" the water in the ocean (not just sodium chloride—but other types of salts), the "DENSER" the water.
    Of course, "temperature" is indirectly related, since warmer water is LESS dense that cold water, and colder water is denser and sinks.

But "density" is DIRECTLY related to "deep-sea ocean currents" and is the correct answer choice.

In the midwest because it has more moderate climate than the north or south west which makes farming far easier.